The present invention relates generally to a feed water preheater.
When superheated steam is introduced into a feed water preheater, in particular into a condensing preheater, part of the superheat can be thermodynamically utilized in a desuperheater, if the steam is sufficiently superheated. The steam is introduced through a branch, pointing towards the tube bundles, into the desuperheater and is passed in a counter-current manner around the tube bundles. The steam accordingly heats up the feed water flowing in the tubes, with the heating taking place by convection. After flowing through the desuperheater, the steam passes into the condensing part of the preheater, where the steam is precipitated.
In feed water preheaters, the steam pressure in the condensing part of the preheater is substantially lower than at the inlet of the desuperheater, because of the flow losses which the steam incurs while passing through the desuperheater until the steam leaves the latter.
In known feed water preheaters, for example as described in German Offenlegungsschrift No. 2,441,324, annular gaps are present between the isolating support plate of the desuperheater and the preheater tubes, where the tubes pass through. The steam can thereby escape through these gaps into the condensing zone, outside of the intended outlet of the desuperheater, or from a steam distribution duct.
In vertical preheaters having an upward steam flow, a layer of condensate is present on the isolating support plate of the desuperheater. Especially in the case of large preheaters, the velocity of the steam through the annular gap can be very high so that droplets of water are carried over from the layer of condensate and are thrown against the tube walls. This water can cause damage by mechanical ablation of material as well as by erosion and by corrosion which can result in the destruction of the tubes.
It is accordingly a primary object of the present invention to provide a feed water preheater in which the pressure difference upstream and downstream of the isolating baffle plate of the desuperheater is reduced and in which the utilization of the steam temperature can be optimized without damage to the plant occuring in the desuperheater.
According to the invention, the abovementioned object is achieved when the warm tube bundle part is divided into at least two part bundles, namely a desuperheater bundle and a condenser bundle and a condensate drain channel is located between the two part bundles.
It is also advantageous to arrange the cross-sections, which are exposed to the steam, of the two part bundles approximately equally. This arrangement results in the advantage that, for example in the layout of feed water preheaters, a standardization of the tube bundles becomes possible.
Depending on the size and capacity of the preheater, however, it can also be advisable for the two part bundles to have cross-sections which are variable relative to one another.
This second arrangement will prove advantageous wherever high steam velocities occur. With such an arrangement, a lower steam rate is then passed through the desuperheater part bundles and a higher steam rate is passed through the condenser part bundle.
According to another preferred embodiment, collection pockets for taking away the condensate from the warm part bundles can be provided between the condensate drain channels and the radial baffle plates.
A particular advantage of the arrangement according to the invention is above all to be seen in the fact that, due to the division of the warm tube bundle part into two part bundles (specifically into an inner part bundle and an outer part bundle), a reduction of the pressure difference upstream and downstream of the support plate of the desuperheater and hence a reduction of the flow velocity through the annular gap are achieved in spite of a relatively high cross-flow velocity of the steam in the desuperheater. At the same time, the steam can leave the desuperheater at a point with favorable flow conditions and enter the central steam distribution duct at that point. As a result of the provision of at least one, but preferably two steam inlets opposite one another which (in particular in the case of U-shaped tube bundles) are arranged perpendicular to the plane of the U-arms, the steam can be passed into the preheater where there are no tubes so that the steam first fills this space and only then flows around the tubes in the desuperheater at a uniform and permissible velocity. In this way, in addition to the prevention of damage, it is also impossible for vibrations to occur.
The fitting of condensate drain channels enables the condensate which forms both on the baffle plates above the desuperheater and also on the isolating support plate to run off. As a result, the heating surface area which would be inactivated by stagnant condensate is available again for the preheating process. Furthermore, a mutual impact of steam and condensate is prevented, and no droplets of water are thus carried over by the steam flow.